Sodium and potassium cyanide are used widely for preparing electrolytic baths and case hardening salt baths and for the synthesis of organic compounds; sodium cyanide and calcium cyanide are used in large amounts for recovering gold by the cyanide leaching of ores. When used for electrolytic baths, the alkali metal cyanides must also have very high purity. Whereas a small concentration of alkali metal hydroxide in alkali metal cyanide acts as a stabilizer, the concentration of alkali metal carbonate and alkali metal formate must be as low as possible. From a safety point of view, cyanides generally have to be available in a dust-free granular form.
It is known that alkali metal cyanides can be prepared by neutralizing hydrogen cyanide (HCN) and alkali metal hydroxide in aqueous solutions followed by crystallization, solid/liquid separation and subsequent mechanical shaping. These types of processes are very costly and the products tend to form dust and are therefore difficult to handle. A particular disadvantage is that some of the mother liquor has to be removed from the process in order to counteract enrichment of by-products and to obtain pure products. Between 10 and 30% of alkali metal cyanide is generally eliminated with the mother liquor.
The industrial process used hitherto for shaping alkali metal cyanides, which is very costly, can be substantially improved by so-called fluidized bed spray granulation, as described in EP-A 0 600 282. In the process mentioned, an alkali metal cyanide solution is sprayed onto a fluidized bed comprising seed granules of alkali metal cyanides and the water introduced is evaporated by means of a stream of drying gas flowing through the fluidized bed. Granules comprising substantially spherical particles which have very low abrasion and a low caking index are obtained. Since an aqueous alkali metal cyanide solution is required in order to perform the process in EP-A 0 600 282 and this is obtained in a known manner by neutralizing HCN with alkali metal hydroxide in aqueous solution, this solution also contains known by-products, including in particular the corresponding carbonate and formate. As a result of using an alkali metal cyanide solution which contains the by-products, the alkali metal cyanide granules prepared therefrom cannot be any purer than the solution itself. Additional alkali metal carbonate is formed due to reaction of the carbon dioxide contained in the fluidizing/drying air with excess alkali metal hydroxide contained in the alkali metal cyanide solution, so that the alkali metal carbonate concentration in the alkali metal cyanide granules is generally greater than that of alkali metal cyanides produced by a crystallization process with subsequent shaping.
A quite different process for preparing solid particulate alkali metal cyanides is known from DE-A 38 32 883. In this process a hydrogen cyanide-containing gas is reacted continuously with finely distributed droplets of an aqueous alkali metal hydroxide solution, while water which is introduced and produced is simultaneously evaporated. Deposited solid particles, after separation, are taken to a shaping and/or post-drying process. This process thus constitutes a spray drying process which is combined with a gas/liquid reaction, here a neutralization reaction. Using this process, therefore, products with a high concentration of alkali metal cyanide are only produced if HCN is used in a large excess and the alkali metal hydroxide has a low concentration. The combination of spray drying and neutralization has the disadvantage that the fine drops start to dry from the outside, which is where the alkali metal cyanide has formed. Diffusion of the HCN gas into the inner core of the droplets, where liquid alkali metal hydroxide solution is still present, becomes difficult as the layer of solid in the outer region becomes thicker. In order to produce a high rate of conversion, therefore, the driving force for diffusion must be high and this is favored by a high excess of HCN and/or a reduction in the drying time. However this reduces the space-time yield for the process. A further disadvantage of this process is that the product is obtained as a fine powder and has to be shaped in additional process steps to provide granules which are safe to handle.
DE-A 38 32 883 mentioned above also discloses that virtually no reaction takes place between solid, finely distributed alkali metal hydroxide and gaseous hydrogen cyanide at the temperatures which are suitable for the process. The reaction requires the presence of water and dissolved alkali metal hydroxide. Taking into account this disclosure, this process cannot be used to prepare alkaline earth metal cyanides, in particular calcium cyanide, by using an aqueous suspension of an alkaline earth metal hydroxide.
Trojosky et. al. in Chem. Ing. Technik September 1995, page 184, describe a process for half-dry flue gas desulphurization by using an absorption drying process in a fluidized bed. In this process, an aqueous calcium hydroxide suspension is applied to the surface of a fluidized bed in a fluidized bed unit using two-fluid nozzles. Although a satisfactory degree of desulphurization of the flue gas is produced by adding the absorption agent in an amount greater than that required stoichiometrically, the proportion of gypsum in the extracted granular end product is too low, since it also contains a considerable amount of calcium oxide or calcium hydroxide in addition to gypsum. Due to the incomplete reaction and thus unsatisfactory purity of the granular material obtained, it is not obvious that the process described in this document can be used for preparing alkali metal cyanides and alkaline earth metal cyanides, in particular alkali metal cyanides of high purity.